1. Field of the Invention
The present invention relates to an optical branching unit, which is usable in the field of optical communication, and outputs a light beam input to an input end to a plurality of output end.
2. Description of the Related Art
In an optical branching unit using an optical waveguide, it is particularly desired as an optical characteristic that the wavelength uniformity of insertion loss is not varied in each branching destination, namely, the wavelength uniformity of each destination is substantially equal.
For example, Jpn. Pat. Appln. KOKAI Publication No. 4-172308 discloses a Y-branch optical circuit, which includes, as a part of an input waveguide, a waveguide whose higher mode is cutoff, and a low equivalent diffractive index in a ground mode is lower than that of the other waveguides formed in a substrate.
However, even in the proposal disclosed in the above Publication No. 4-172308, the wavelength uniformity varies in each branching destination when a light beam guided to the input waveguide is displaced from the axial line or a center axis of the input waveguide. The wavelength characteristic is particularly lowered in a shorter wavelength side.
Further, the input waveguide of an optical branching unit is connected to an optical fiber, for example, and an optical signal transmitted in the optical fiber is applied (input). In many cases, an input waveguide is glued to an optical fiber, and it is difficult to completely prevent displacement between the waveguide axial line and the optical fiber center axis, during gluing the waveguide and optical fiber or hardening the glue.